In the past, single compressor and dual evaporator automobile air conditioning systems have been used. These systems typically used a thermally responsive expansion valve and a suction throttling type flow control valve for regulating refrigerant flow and to maintain evaporator pressures sufficiently high to prevent frost accumulation on the evaporators. These suction throttling valves are expensive and often two were used in the dual evaporator systems. In a modern automobile air conditioning system a simple orifice or capillary type expander is used in place of the thermal expansion valve and a refrigerant pressure responsive switch to alternate compressor activation and deactivation is used in place of the suction throttling valve.
A current Ford Motor Company vehicle has a single compressor and dual evaporator air conditioning system using a refrigerant pressure switch for cycling the compressor off and on. A first evaporator is located in the forward portion of the automobile interior and a second evaporator is located in the rearward portion. The control switch to cycle the compressor is associated with the first evaporator. One fan is provided for each evaporator to pass air through and over the evaporator's surfaces.
The above described air conditioning system performs satisfactorily when both fans are active because the refrigerant pressure in each evaporator is substantially equal under this operative condition. However, a problem develops when the operator of the automobile elects an air conditioning mode in which the rear fan is off while the front fan is on. In this operative mode, the capillary type flow control allows some refrigerant flow into the rear evaporator while no air is passed over the evaporator's surfaces due to the inoperative fan. Resultantly, a very low temperature and pressure is created in the rear evaporator.
When the refrigerant pressure responsive switch senses a predetermined low refrigerant pressure near the outlet of the front evaporator, the switch deactivates the compressor as planned and expected. Thereafter, the very low pressure which is generated in the rear evaporator causes refrigerant to immediately start to flow from the outlet of the front evaporator into the conduit leading to the rear evaporator. This reverse flow further lowers the refrigerant pressure about the switch and fools the switch into unduly prolonging the next reactivation of the compressor. Meanwhile, a continuing stream of warm air from the automobile interior passes through the front evaporator and warms its surfaces so that the discharge temperature of air is undesirably high. Resultantly, a vehicle occupant is made uncomfortable by the large "swing" or temperature gradient between the lowest and the highest temperature discharged from the front evaporator.
The solution used by Ford to solve this potential problem is to locate a shut-off valve in the refrigerant conduit or line which feeds liquid refrigerant to the rear evaporator. This valve then positively blocks flow to the rear evaporator whenever the rear fan is not activated. As a consequence, the very low refrigerant temperature and pressure previously identified is not generated in the rear evaporator. However, there is still a significant volume in the relatively long rear suction conduit into which refrigerant will flow when the rear fan is turned off. Thus this fix only partially solves the back flow problem and the electrically controlled positive shut-off valve is costly.